pub struct EarlyOtherwiseBranch;
Expand description
This pass optimizes something like
ⓘ
let x: Option<()>;
let y: Option<()>;
match (x,y) {
(Some(_), Some(_)) => {0},
_ => {1}
}
into something like
ⓘ
let x: Option<()>;
let y: Option<()>;
let discriminant_x = std::mem::discriminant(x);
let discriminant_y = std::mem::discriminant(y);
if discriminant_x == discriminant_y {
match x {
Some(_) => 0,
_ => 1, // <----
} // | Actually the same bb
} else { // |
1 // <--------------
}
Specifically, it looks for instances of control flow like this:
=================
| BB1 |
|---------------| ============================
| ... | /------> | BBC |
|---------------| | |--------------------------|
| switchInt(Q) | | | _cl = discriminant(P) |
| c | --------/ |--------------------------|
| d | -------\ | switchInt(_cl) |
| ... | | | c | ---> BBC.2
| otherwise | --\ | /--- | otherwise |
================= | | | ============================
| | |
================= | | |
| BBU | <-| | | ============================
|---------------| | \-------> | BBD |
|---------------| | | |--------------------------|
| unreachable | | | | _dl = discriminant(P) |
================= | | |--------------------------|
| | | switchInt(_dl) |
================= | | | d | ---> BBD.2
| BB9 | <--------------- | otherwise |
|---------------| ============================
| ... |
=================
Where the otherwise
branch on BB1
is permitted to either go to BBU
or to BB9
. In the
code:
BB1
isparent
andBBC, BBD
are childrenP
ischild_place
child_ty
is the type of_cl
.Q
isparent_op
.parent_ty
is the type ofQ
.BB9
isdestination
All this is then transformed into:
=======================
| BB1 |
|---------------------| ============================
| ... | /------> | BBEq |
| _s = discriminant(P)| | |--------------------------|
| _t = Ne(Q, _s) | | |--------------------------|
|---------------------| | | switchInt(Q) |
| switchInt(_t) | | | c | ---> BBC.2
| false | --------/ | d | ---> BBD.2
| otherwise | ---------------- | otherwise |
======================= | ============================
|
================= |
| BB9 | <-----------/
|---------------|
| ... |
=================
This is only correct for some P
, since P
is now computed outside the original switchInt
.
The filter on which P
are allowed (together with discussion of its correctness) is found in
may_hoist
.
Trait Implementations
sourceimpl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch
impl<'tcx> MirPass<'tcx> for EarlyOtherwiseBranch
sourcefn is_enabled(&self, sess: &Session) -> bool
fn is_enabled(&self, sess: &Session) -> bool
Returns
true
if this pass is enabled with the current combination of compiler flags.fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>)
fn name(&self) -> Cow<'_, str>
sourcefn phase_change(&self) -> Option<MirPhase>
fn phase_change(&self) -> Option<MirPhase>
If this pass causes the MIR to enter a new phase, return that phase.
fn is_mir_dump_enabled(&self) -> bool
Auto Trait Implementations
impl RefUnwindSafe for EarlyOtherwiseBranch
impl Send for EarlyOtherwiseBranch
impl Sync for EarlyOtherwiseBranch
impl Unpin for EarlyOtherwiseBranch
impl UnwindSafe for EarlyOtherwiseBranch
Blanket Implementations
sourceimpl<T> BorrowMut<T> for Twhere
T: ?Sized,
impl<T> BorrowMut<T> for Twhere
T: ?Sized,
const: unstable · sourcefn borrow_mut(&mut self) -> &mut T
fn borrow_mut(&mut self) -> &mut T
Mutably borrows from an owned value. Read more
Layout
Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...)
attributes. Please see the Rust Reference’s “Type Layout” chapter for details on type layout guarantees.
Size: 0 bytes